Various chemical processes use precise quantities of materials which must be introduced as vapors into a processing chamber at precisely controlled rates in order to yield a product of a defined composition. For example, in the microelectronics industry, controlled amounts of vaporized precursor materials must be carefully introduced into a chemical process chamber for reaction with other materials and for deposit onto a wafer (such as by condensation). In this and other chemical processes, such as chemical etching, stripping, and passivation, the ability to measure and control the flow of each vaporized precursor is vital for reliable manufacture of products having consistently high performance quality.
Vapors of precursor materials used to manufacture microelectronic and semiconductor devices are typically derived from liquid or solid precursor materials. A suitable delivery pressure for a reactant vapor depends on the vapor pressure of the liquid or solid precursor from which the vapor is derived, as well as the particular requirements of the mass flow controller.
Vapor-phase reactant delivery in about ninety percent of current microelectronics processing applications can be effectively controlled using so-called thermal mass flow controllers. With a thermal mass flow controller, one can determine the rate of laminar fluid flow in a heated channel by measuring the temperature losses from the fluid as it flows through the channel. Temperature measurement of the fluid can be accomplished with any known temperature measuring device, such as, for example, thermistors, thermocouples, and the like. For processes which require operation in a pressure range of 10.sup.-4 torr through atmospheric pressure (760 torr), a flow controller inlet pressure range of 50 torr to several atmospheres, fluid temperatures ranging from ambient through about 40.degree. C., and mass flow rates of 1.times.10.sup.-3 to 1000 torr-liters/sec, thermal mass flow controllers are highly effective in delivering a controlled flow of vapor to the process reactor.
For delivery of precursor materials to process reactors in systems in which the operating temperatures, pressures and flow rates are outside of these ranges, other devices for controlling mass flow may be more useful or effective. For example, a bubbler may be used to bubble a carrier gas, such as hydrogen or helium, through a source of a heated liquid reactant which is typically provided in a temperature-controlled ampule. The carrier gas bubbles through the liquid reactant in the ampule and becomes saturated with the vapor of the reactant material. It is then delivered to the reaction chamber through heated lines.
A disadvantage of bubblers is the periodic need to refill the ampule with reactant liquid. Replenishment of the ampule with liquid reactant introduces contaminants into the process, as well as batch-to-batch variations in various properties of the material, such as, for example, the viscosity and vapor pressure. The refilling process is messy, time-consuming and labor-intensive, and therefore costly. In addition, as the liquid level in the ampule changes, the ratio of carrier gas to reactant vapor changes also, making it difficult to control the amount of reactant delivered to the process chamber. Another disadvantage is that the bubbler requires a relatively large volume of reactant material which must be maintained at an elevated temperature. Some reactant materials undergo unfavorable chemical reactions or become otherwise unstable if held for prolonged periods of time at the desired maintenance temperatures.
An alternative to bubblers involves the use of a heated ampule of a liquid or solid reactant material. The vapor from the heated reactant material is provided directly to the inlet of a mass flow controller at an appropriate vapor pressure, without a carrier gas. This technique avoids the precise temperature and pressure control requirements of a bubbler and requires only that the reactant temperature be sufficient to deliver the vapor to the mass flow controller at a suitable pressure.
Disadvantages of this technique include those mentioned above relating to replenishment of the reactant material and the need for a relatively high volume of material to be held at an elevated temperature for potentially lengthy periods. In addition, this technique may not be effective for a reactant material whose vapor pressure at the ampule maintenance temperature is relatively close to the total pressure of the reaction chamber. The mass flow controller cannot effectively deliver fluid unless there exists a sufficient pressure differential across it.
Another alternative is to use a liquid flow control device, such as a volumetric displacement pump or a liquid mass flow controller in combination with a reservoir of liquid reactant material, with a downstream vaporizer. The liquid reactant material is provided at substantially ambient temperature, from either a reservoir or an integral distribution system, to a flash vaporizer, and the vaporized reactant is delivered to a process reactor. No temperature control is required for the liquid before it enters the vaporizer.
A disadvantage of this approach is the requirement of a flash vaporizer. In order to deliver a nonfluctuating, controlled flow of a reactant vapor to the process reactor, the vaporizer must operate as a continuous liquid-to-vapor conversion device which responds virtually instantaneously to flow rate changes. In addition, there may be deposition of particles from within the liquid reactant material and the formation of residual films on various vaporizer surfaces during the vaporization process. These events may lead to clogging of the vaporizer and delivery lines and may necessitate their frequent flushing and cleaning.
Alternative devices have been developed to measure and control the mass flow of vapors not suited for measurement by thermal mass flow transducers or the other devices discussed above. These are typically pressure-based mass flow controllers which measure temperature and pressure of a fluid flowing around and/or through a flow restrictor, such as a laminar flow element.